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I Get. 9, 1928 v 1,686,901

c. A. CULYER MICROPHONE Filed July 1, 1927 2 Sheets-Sheet 1 IN V EN TOR.

BY 6- 0g 'ATTORNEYV.

MICROPHONE Filed July 1, 1927 2 Sheets-Sheet 2 [Ear/e5 A. [El [061;

. BY d AITTORNEY IN VEN TOR.

Patented Oct. 9, 1928.

UNITED STATES PATENT OFFICE.

cnaamas A. cvnvnn, or NOBTHFIELD, mmmiso'ra, assronon 'ro wmnn mmo, 1nd, on NEW YORK, n- Y., A conrorwrron or DELAWARE.

, mcnornonn.

Application filed my 1 1921. Serial 110. 202,962.

My invention relates to an improved type of microphone particularly useful in connection with radio broadcastlng and public address systems.

One of the objects of my invention is to provide a microphone or pick-up device in which resonant effects are eliminated.

Anotherobject of my invention is to provide a microphone construction in which a linear energy response may be obtained.

A further object of my invention is to provide a microphone of a portablecharacter haying a high degree of sensitivity and permanency of adjustment- My invention will be more clearly understood from the specification hereinafter following by reference to the accompanying drawings in which:

Figure 1 is a cross-sectional view taken through a microphone showing one form of my invention; Fig. 2 is a rear end view of the microphone construction illustrated in Fig. 1; Fig. 3 is a front elevation of the microphone construction illustrated in Fig. 1; Fig. 4v is a cross-sectional view of a microphone embodying the rinciplcs of my in ention;

and Fig. 5 is a rent elevation of the microphone of the construction illustrated in Fig. 4.

Referring to the accompanying drawings, the body of the unit designated as 11 consists of a block of heavy material such" as brass which forms a non-.vibratile form and support for the several electrical and inechanical elements constituting the devlce. The conical chamber 1 has both its larger and smaller openin s closed by means of thin and highly flexi 1e material such as rubber or gold-beaters skin, these two membranous walls being designated as 2 and 2' in the diagram. These flexible walls are held in position by suitable cords or hands 3 which rest in appropriate grooves. For additional security the flexible ends may be cemented to the edge of body 11. The space thus formed by the body 11 and the two flexible walls 15 com letely filled with a liquid such aswater, alco 01, or glycerine. To the smaller of the two flexible walls is attached, by means of suitable adhesive material, a layerofextreme- 'ly thin gold foil 5. A small cell or r ece tacle 7 made of insulating material is eld rigidly in position by means of the attached shank 14, this shank being firmly supported by the metal bridge eonsistlng of the parts '4, 10 and 8. The cup or receptacle 7 is suported in contact with the com osite flexl ble wall 5. Within the cell 7 and 1s placed theusual olished carbon disk 9.0r othersimilarmicrop onicsurface. The space between the flexible gold surface 5 and the carbon mirror 9 is nearl filled with granular carbon 6 or other simi ar material suitable for m 1crophon ic purposes. Suitable flexible electrical terminals 12 and 13 are attached to the bridge bar 8 and to the old surface o whereby the microphone may e connected in series with the usual source of potential and the primary of a transformer.

The operation of the microphone of my invention is as follows: Sound waves impinging on the flexible wall 2 give rise to a varymg pressure, which pressure, as a result of the operation of Pascals principle, is transferred without loss to the rear flexible wall 2', and thence to the granular material 9. As a result of this variable pressure on the microphonic elements in the cell the energy of the incident sound waves is converted into a variable electrical current in a manner well known to the art.

Owing to the fact that the two end walls of the chamber are highly flexible and, in general, under difl'erent peripheral tension, and because of the fact that the liquid between said membranous walls possesses a relatively large mass, damping is complete and the moving system is therefore aperiodic. As a result of this mechanical property resonant efl'ects with the attendant distortion are eliminated, thus overcoming one of the inherent defects in former devices of this character.

In other microphonic pick-up devices (except in the caseof the device disclosed in in Patent 1,616,414) an effort is made to mimmize undesirable resonance effects by subjecting the diaphragm to high peripheral tension. Such an arrangement results in comparatively low sensitivity and also in a nonlinear energy response. In my invention the membranes are hlghly flexible and are under relativel small peripheral tension. As a result of t ese conditions the sensitivity of the organization to sound waves is relatively high, and the energy response proves to be approximately uniform over a wide range of frequencies. i

Another advantage residing in m invention is. that while the device 1s high sensitive and also free from'resonant e sets, it

at its back sound waves, with the result that a more nat ural tonal reproduction is secured.

Figs. 4 and 5 show another one of the several possible practical forms which my invention may take. This arrangement provides for two microphonic cells 7 instead of one, as above described, thus making it possible to utilize a push pull or balanced microphone circuit. The conical chamber 11- is here replaced by a cylindrical enclosure 18, both ends of which are closed by the flexible membranes 2 and 2'. Attached to each membrane is a very thin layer of gold foil, and the two microphonic cells 7 are supported as shown. port-ed centrally within the frame 10 by means of screw devices 15. The bridge members 8 extend diametrically across opposite ends of 'the frame 10 carrying the microphonic cells, which are-supported by the members 14 projecting through the bridge members 8. The members 14 are each provided with a flange 16 to which the microphonic cell. is attached. Sound waves enter opposite ends of the frame 10 and impinge against the flexible sheets of foil 5 and the membranes 2 and 2 secured bymeans of cords 3 over opposite ends of the cylindrical frame 18.

The construction of microphone as described herein has proven highly sensitive and yet the apparatus is practical and readily portable. Flexible membranes are placed under moderate peripheral tension. -The mass of liquid between the membranes possesses relatively great inertiaand the moving sys- The cylindrical member 18 is sup tembecomes aperiodic, of relatively great closing opposite ends of saidchamber, metallic foils carried by the exterior of said flexible walls, a mierophonic device positioned in electrical contact with said metallic foils and located with respect to said flexible walls for receiving forces incident thereon from said liquid content corresponding to pressures derived from sound waves impinging against said flexible walls.

2. A microphone comprising a frame structure, a chamber open at opposite ends thereof and disposed within said frame structure, a flexible wall closing each end of said chamber, a metallic coating extending over the exterior of each of said flexible walls, a microphonic contact disposed at substantially the center of each of said flexible walls and resting in contact with the metallic coatings the reon, and a fluid mass disposed within said chamber and between said flexible walls whereby the pressure efiects oi'sound waves inc dent upon one of said flexible walls are transmitted through said fluid mass and against each of said microphon i c contacts for subjecting said microphonic contacts to varia ble pressure in accordance with variations in sound.

In testimony whereof I aflix my signature.

. CHARLES A. CULVER. 

